Magnetic heading that is produced by current inertial navigation systems (INS) is not meeting predefined required accuracy specifications. This can be an issue when an aircraft is trying to align itself with a runway because runway headings are typically provided as a magnetic heading.
The magnetic heading provided by current INSs is synthesized from INS true heading, INS position, and magnetic variation (magvar) that is derived from a static lookup table (LUT). The magnetic heading accuracy specification (e.g., TSO-C6d; AS8013) states that the magnetic heading will meet a given accuracy (typically 2 to 3 degrees) for a given duration in time (typically 10 years) for the specified magvar LUT.
A static magvar LUT can prevent the INS from meeting the magnetic heading accuracy specifications for dates sufficiently far from the magvar LUT optimization date. This has become more common in recent years due to the drift of the location of the north magnetic pole, the geometry of the magvar LUT polar exclusion areas (FIG. 2-1), and the high rate-of-change of the magnetic variation near the polar exclusion areas (FIG. 2-2).